The 5 key UNSC members also press Tehran to agree with UN nuclear watchdog on access to "relevant sites and information"; call on N. Korea to refrain from new nuke tests.

The five permanent members of the UN Security Council put pressure on Iran on Thursday to allay international concern about its nuclear program, and said they expected talks with Tehran to lead to concrete steps toward a negotiated solution.

In a joint statement issued at a nuclear meeting in Vienna, the United States, France, Russia, China and Britain pressed Tehran to agree urgently with the UN nuclear watchdog on access to "relevant sites and information."

Western diplomats say Iran appears to be stonewalling a request by the International Atomic Energy Agency (IAEA) for access to a key military site, Parchin, where it believes military-related nuclear research may have taken place.

Iran and major powers resumed talks in April in Istanbul after more than a year - a chance to ease growing tension and help to avert the threat of a new Middle East war - and they will meet again on May 23 in Baghdad.

Thursday's statement by the five powers - which together with Germany are involved in nuclear talks with Iran - said they were seeking a "sustained process of serious dialogue," where both sides can take urgent practical steps to build confidence.

"We expect that subsequent meetings ... will lead to concrete steps toward a comprehensive negotiated solution which restores international confidence in the exclusively peaceful nature of the Iranian nuclear program," it added.

The five permanent members also called on North Korea to refrain from any new nuclear tests.

North Korea, which tested plutonium devices in 2006 and 2009, has almost completed preparations for a third nuclear test, a senior source with close ties to Pyongyang and Beijing told Reuters last month.

"We ... call on (North Korea) to refrain from further actions which may cause grave security concerns in the region, including any nuclear tests," said in the joint statement.